You could try a different type of hay. Some buns are very picky with hay. I have a bunny with a sensitive tummy, he can have leafy greens and fennel though. His tummy is always better in the summer when I can forage for him. Stuff he likes and that agrees with him are apple leaves and twigs (from apple trees that haven’t been treated with pesticides), willow and rowan. Some buns like raspberry or blackberry leaves. What you can get of course depends on where you live. If there are racoons and wild bunnies near you, it’s not recommended to pick stuff that’s growing on the ground, due to the risk of parasites transmitted through the wild animals’ feces and urine. Otherwise plain grass is fine, many buns prefer that to hay and it’s a very species-appropriate food for them.
Bam’s big favorite just now is dill. Dill is traditionally used for tummy problems in humans, as is mint.
Rabbits do have varying sensitivities towards food. So what one bun might thrive on, the next bunny may get sticky poop from. Sometimes a probiotic help. Sticky poop is the result of some disturbance to the GI microbiota, and there’s most likely some genetic component as well that makes certain buns prone to ISC (intermittent soft cecotropes), as the condition is called.
I’m guessing you have read up on this problem online, but I’ll still paste links to two good articles on sticky poop:
http://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
http://www.bio.miami.edu/hare/poop.html